1. Field of the Invention
The present invention relates to a hydrostatic stepless transmission (HST). More particularly, it relates to a mechanism for easing rising of hydraulic pressure at the time of sudden speed change operation of the HST.
2. Related Art
It is well known that a conventional vehicle having an HST operated by a speed change operation member, such as a lever or pedal, is provided with a mechanism for softening output change of the HST as to prevent shock or jerky movement, that is, lifting of either the front or rear portion of the vehicle when the speed change operation member is suddenly operated to start or stop the vehicle.
For example, in a conventional vehicle as disclosed in the Japanese Utility Model Laid Open Gazette Sho. 61-32653, a shock absorber filled with fluid or gas is disposed outside a transmission housing incorporating an HST and is connected to a linkage between a speed change manipulator (lever) and an operation arm for changing output of the HST, so as to soften movement of the operation arm even when suddenly operating the speed change lever. The connection between the hydraulic cylinder and the linkage is complicated. The external hydraulic cylinder must prevent fluid or gas from leaking and guard against water, dust, corrosion and the like, thereby making it expensive.
Furthermore, since rotation of the operation arm is slowed, acceleration of the vehicle may be unexpectedly slowed. More specifically, even if the shock absorber is set to apply operational weight onto the speed change lever such as being suitable for some people, other people may feel it is difficult to operate the speed change lever to get appropriate acceleration, and other people may apply a heavier operational force onto the speed change lever against the resistant force of the shock absorber, thereby unexpectedly suddenly changing output speed of the HST and causing shock. Thus, it is hard to appropriately set resistance of the shock absorber for operational force.
A conventional transaxle as disclosed in the Japanese Utility Model Laid Open Gazette Hei. 3-69755 has a housing incorporating an HST and filled therein with oil. A hydraulic cylinder is disposed in the housing and supplied with oil from the oil sump in the housing. The hydraulic cylinder is connected to a linkage for moving a movable swash plate of the HST. However, the hydraulic cylinder serves as a shock absorber for softening movement of the swash plate, thereby including the same problem as mentioned above.
A conventional transaxle as disclosed in the Japanese Patent Laid Open Gazette Sho. 59-216733 has a switching valve which can be opened and closed by operating a speed change operation means for changing output rotational speed or direction of an HST. The switching valve is provided for expanding a neutral zone of the HST so that, when the speed change operation means is moved adjacent to its neutral position, the switching valve is opened to bypass between the higher-pressure area and the depressed area in the HST closed circuit. The switching valve is disposed outside a housing incorporating the HST, thereby having the above-mentioned problem in its protection.
A conventional transaxle as disclosed in U.S. Pat. Nos. 5,836,159 and 6,109,032 has a housing forming an oil sump therein, in which an HST comprising a variable displacement hydraulic pump, a hydraulic motor, and a center section with a closed circuit formed therein for fluidly connecting the hydraulic pump to the hydraulic motor are housed. In the housing, a rotary member as a part of a linkage from a speed changing operation device is connected to a movable swash plate of the hydraulic pump. A valve member, fluidly connected to the closed circuit, is fitted in the center section, and projects outward from the center section. The rotary member is extended to form a plate-like portion in contact with the outer end of the valve member. The plate-like portion is formed with a groove. When an opening of the valve member communicates with the groove, oil is drained from the closed circuit to the oil sump in the housing so as to expand the neutral zone of HST, thereby easing shock when the vehicle starts or stops. Thus, the opening-and-closing of the valve member depends on rotation of the plate-like portion of the rotary member in contact with the opening end of valve member, thereby requiring a considerably large space for rotation of the plate-like portion in the housing.
Especially, each of the switching valve of the Japanese Document No. '733 and the mechanism of the U.S. Document Nos. '159 and '032 expands the neutral zone of HST so as to ease shock in case of starting or almost stopping a vehicle. However, oil leakage from the HST closed circuit is prevented when the HST is set out of the expanded neutral zone. Thus, when there is a sudden speed change operation of the HST out of the neutral zone, for example, when the speed change operation device held in a certain speed position is moved suddenly, or when there is a sudden speed change operation that increases the output speed of the HST beyond the speed range corresponding to the expanded neutral zone, the sudden speed change operation is directly reflected in output change of HST, thereby causing a shock.